1. Field of the Invention
The present invention relates to a can type secondary battery, and more particularly to a can type secondary battery having a fixing unit which improves the safety of the secondary battery.
2. Description of the Prior Art
As portable wireless appliances such as video cameras, portable phones, and portable computers have become more compact and lightweight with high-grade functions, various studies are being carried out in relation to secondary batteries which are used as power sources for such portable wireless appliances. Secondary batteries include Ni—Cd batteries, Ni-MH batteries, Ni—Zn batteries and lithium secondary batteries. Among other things, lithium secondary batteries are rechargeable batteries fabricated in a small size with high capacity. The lithium secondary batteries represent high operational voltage and high energy density per unit weight, so lithium secondary batteries are extensively used in advanced electronic technology fields.
FIG. 1 is an exploded perspective view illustrating a conventional can type lithium ion secondary battery.
According to the conventional can type lithium ion secondary battery, an electrode assembly 112 including a second electrode plate 113, a first electrode plate 115 and a separator 114 is accommodated in a can 110 together with an electrolyte. An upper opening section 110a of the can 110 is sealed by means of a cap assembly 120.
The cap assembly 120 includes a cap plate 140, an insulation plate 150, a terminal plate 160 and an electrode terminal 130. The cap assembly 120 is accommodated in an insulation case 170 and is coupled to the upper opening section 110a of the can 110 to seal the can 110.
The cap plate 140 is a metal plate having a size and a shape corresponding to the upper opening section 110a of the can 110. The cap plate 140 has formed a first terminal hole 141 at its center having a predetermined size into which the electrode terminal 130 is insertable. A gasket tube 146 is coupled around the electrode terminal 130 in order to insulate the electrode terminal 130 from the cap plate 140 when the electrode terminal 130 is inserted into the first terminal hole 141. In addition, an electrolyte injection hole 142 having a predetermined size is formed at one side of the cap plate 140. After the cap assembly 120 has been assembled with the upper opening section 110a of the can 110, the electrolyte is injected into the can 110 through the electrolyte injection hole 142. Then, the electrolyte injection hole 142 is sealed by means of a sealing member.
The electrode terminal 130 is connected to a first electrode tap 117 of the first electrode plate 115 or a second electrode tap 116 of the second electrode plate 113 in such a manner that the electrode terminal 130 may act as a first electrode terminal or a second electrode terminal.
The first electrode plate 115 may be used as a positive electrode plate or a negative electrode plate. Additionally, the second electrode plate 113 may also be used as a negative electrode or a positive electrode plate.
The insulation plate 150 may be made from insulative material substantially similar to the material used for a gasket and coupled to a bottom surface of the cap plate 140. The insulation plate 150 may be formed at a predetermined portion thereof with a second terminal hole 151, which corresponds to the first terminal hole 141 of the cap plate 140. The electrode terminal 130 may extend through the second terminal hole 151 of the insulation plate 150. In addition, the insulation plate 150 may have a bottom surface formed with a resting groove 152 having a size corresponding to a size of the terminal plate 160 in such a manner that the terminal plate 160 can be stably rested in the resting groove 152.
The terminal plate 160 is made from a Ni-alloy and is coupled to the bottom surface of the insulation plate 150. The terminal plate 160 includes a third terminal hole 161 corresponding to the first terminal hole 141 of the cap plate 140. Since the electrode terminal 130 may extend through the third terminal hole 161 of the terminal plate 160 while being insulated from the terminal plate 160 by the gasket tube 146, the terminal plate 160 may be electrically insulated from the cap plate 140 and may be electrically connected to the electrode terminal 130.
However, in such a lithium secondary battery, voltage may suddenly rise if an internal short circuit, an external short circuit or overcharge/over-discharge of the electrode assembly occurs. In this case, the lithium secondary battery may cease to function. In order to prevent the secondary battery from short circuiting, insulative tapes are attached not only to end portions of a positive electrode plate and a negative electrode plate of the electrode assembly, but also to a welding section of an electrode tap. In addition, the secondary battery is electrically connected to safety devices, such as a positive temperature coefficient (PTC) element, a thermal fuse and a protecting circuit. Such safety devices may shut off current when the voltage or temperature of the secondary battery suddenly rises, thereby preventing the secondary battery from being broken or damaged.
If the lithium ion secondary battery is deformed due to external impact or external pressure applied thereto, the protective circuit or the protective device may not be able to prevent a short circuit between electrodes. According to a longitudinal compression evaluation method, which is one of the methods for evaluating the safety of the can-type secondary battery, the short circuit between the electrode plates within the can-type secondary battery is a problem. In a longitudinal compression test, which is one of the items for evaluating the safety of the can-type secondary battery, a compression jig is used to compress both lateral surfaces of the can-type secondary battery in a direction perpendicular to the longitudinal direction of the can-type secondary battery. During the compression, the compression surfaces of the compression jig remain parallel to both lateral surfaces of the can-type secondary battery and the compression force is 13 kN. As the can-type secondary battery is compressed according to the longitudinal compression evaluation method, the first and second electrode plates are short-circuited and currents flow abruptly from the second electrode plate to the first electrode plate. As a result, excessive heat is generated by the first and second electrode plates' own resistance. The excessive heating may cause the second battery to explode.
In addition, if the secondary battery is pressed in a longitudinal direction or if a downward external force is applied to the secondary battery from the upper end of the cap assembly, the terminal plate coupled to the bottom surface of the cap assembly may separate from the insulation plate and bend towards the electrode assembly, thereby causing a short circuit between the second electrode plate and the first electrode plate. Therefore, the safety of the secondary battery may be reduced.